[RFC 02/22] configfs: Add struct configfs_item_operations->check_link() in configfs_unlink()

["Nicholas A. Bellinger" <nab@linux-iscsi.org>]

From: Nicholas Bellinger <nab@linux-iscsi.org>

This patch adds a optional struct configfs_item_operations->check_link() check
called in fs/configfs/symlink.c:configfs_unlink() that can be used by configfs
consumers to check for an explict struct config_group dependence with active
symlink and fail with -EPERM before the unlink(2) syscall is allowed to occur.

Currently without this patch, there is not a method that a consumer can tell
configfs_unlink() that it needs to fail for this particular case.  Allowing
->check_link() to propigate up the errno to VFS is also another option for the
call, but currently for TCM using the existing -EPERM in configfs_unlink() is fine here.

Note this patch is used by TCM v4 generic configfs fabric module infrastructure to
allow  explict Initiator Port MappedLUNs symlinks to create a dependency for
the fabric TPG Port LUNs living in a configfs group that is not a direct
struct config_group parent.

Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
---
 fs/configfs/symlink.c    |   13 +++++++++++++
 include/linux/configfs.h |    1 +
 2 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 0f3eb41..b8010de 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -205,6 +205,19 @@ int configfs_unlink(struct inode *dir, struct dentry *dentry)
 	parent_item = configfs_get_config_item(dentry->d_parent);
 	type = parent_item->ci_type;
 
+	/*
+	 * See if the underlying struct config_item has dependent
+	 * symlinks, and should return -EPERM here.
+	 */
+	if (type && type->ct_item_ops &&
+	    type->ct_item_ops->check_link) {
+		if (type->ct_item_ops->check_link(parent_item,
+					sl->sl_target) != 0) {
+			config_item_put(parent_item);
+			goto out;
+		}
+	}
+
 	spin_lock(&configfs_dirent_lock);
 	list_del_init(&sd->s_sibling);
 	spin_unlock(&configfs_dirent_lock);
diff --git a/include/linux/configfs.h b/include/linux/configfs.h
index ddb7a97..7c01d86 100644
--- a/include/linux/configfs.h
+++ b/include/linux/configfs.h
@@ -226,6 +226,7 @@ struct configfs_item_operations {
 	ssize_t	(*show_attribute)(struct config_item *, struct configfs_attribute *,char *);
 	ssize_t	(*store_attribute)(struct config_item *,struct configfs_attribute *,const char *, size_t);
 	int (*allow_link)(struct config_item *src, struct config_item *target);
+	int (*check_link)(struct config_item *src, struct config_item *target);
 	int (*drop_link)(struct config_item *src, struct config_item *target);
 };
 
-- 
1.7.2.2